Microsoft has released Hotfix Updates (HUs) that enable support for new functionality and address issues in the March 2024 Security Update (SU).
The April 2024 HU is available for the following bui...
Updated May 06, 2024
Version 4.0
Blog Post
ceantuco , posts about Download Domains not working with the April24HU seem to have started last night (aside from my issue/non-issue post on Tuesday, 4/23). Prior to that, sentiment in this thread seems to indicate that Download Domains is (was) working after the April24HU.
re: waiting to install April24HU, Microsoft's stance still seems to be "If you did not install March SU, then you should install April HU, as it will address security payload from March 2024. We always want you to run the latest security bits." - Nino_Bilic . Though I understand being skittish after the March24SU...
Do you already have Download Domains enabled (prior to applying March24SU or April24HU)?
[PS] C:\Windows\system32>Get-OrganizationConfig | fl EnableDownloadDomains
EnableDownloadDomains : True
If so and it's working (in OWA: you can see inline images, you can download attachments, and you can preview attachments like PDFs 
), then one assumes (famous last words) you should not have an issue with the April24HU. Regardless, you should be able to uninstall it if it does not function properly ("HUs, like SUs, can be uninstalled"). If your Exchange Server[s] are virtualized, I recommend taking a snapshot before updating Exchange, then delete the snapshot only after everything is installed and basic smoke testing passes.